Hermetically Sealable, Easy-Opeanable, Flexible Container of Heat-Shrinklable Thermoplastic Material

ABSTRACT

A plastic bag for the manufacture of an easy-openable, hermetically sealed package, comprises a multi-layer heat-shrinkable film folded and/or heat-sealed to itself to form a flexible container with two closed side edges, a closed bottom side edge and an open top side, wherein said multi-layer heat-shrinkable film comprises a first outer heat-sealing layer (a), a second outer layer (b) and directly adhered to the heat-sealing layer (a) an internal layer (c) of a blend of at least two resin components that are only partially compatible so that said layer (c) will fail when a transversal force of from about 4 to about 9.5 N/25.4 mm is applied thereto. Preferably the bag is an ES bag and comprises an unsealed area extending beyond the factory bottom seal where the front and rear panels of the bag are separately graspable and usable as tabs to be pulled apart to initiate opening of the package. 
     Also claimed is the packaged product comprising said plastic bag.

FIELD OF THE INVENTION

This invention relates to a flexible container of heat-shrinkablethermoplastic material, such as a bag or pouch, that can be sealed in atight, hermetic manner, to encase a product, providing for a packagethat can withstand the conventional handling without accidentallyrupturing and thus safely securing the packaged product, and at the sametime can be opened by hand, i.e., without using scissors, knives, orother cutting and dangerous devices, when this is desired.

BACKGROUND OF THE INVENTION

Food and other items are often packaged in flexible containers ofthermoplastic material, such as pre-formed bags with one end openthrough which the product to be packaged is inserted into the bag, orpouches that are formed starting from a flat or folded film sealedaround the product to be packaged, which are then closed by heat-sealingthe open end(s).

Particularly with food products, the flexible container is very oftenmade of heat-shrinkable thermoplastic material. In such a case, firstthe product is loaded into the flexible container, then air is removedand the open end of the container is closed by a heat-sealing step, andfinally the sealed and vacuumized package is submitted to aheat-treatment to get the shrink of the packaging material tightlyaround the packaged product. While the opening of plastic bags andpouches is a problem that in general needs improving, in case ofvacuumized and shrunk bags opening of the end package to reach thepackaged product may become a real problem if no cutting tools areavailable. It is therefore desirable to provide the flexible containerwith so-called easy-opening means, i.e., a feature or a combination offeatures that would enable the end user to easily open the package byhand.

U.S. Pat. No. 3,516,537 addresses this problem by creating a tab in theskirt of a heat-shrinkable bag extending beyond the factory seal of thebag, by means of a cut at a right angle to said factory seal. To openthe package the tab is gripped with the fingers of one hand and pulledup and across the bottom of the package, while the packaged product isheld with the other hand. The entrance edge of the tab, being directedat a right angle to the seal, will tear into and through the factoryseal. As the tab is pulled across the package, the package will tearopen predominantly following the sealed seam. This solution however canbe employed only with products that would not be damaged by a certainpressure (such as the pressure exerted by pulling up the tab with onehand while keeping down the product with the other) such as the turkeyspecifically illustrated in U.S. Pat. No. 3,516,537.

A similar approach, with similar drawbacks, has been described in U.S.Pat. No. 3,641,732, where a laminated tear tab extending outwardly andsubstantially perpendicular to the package is formed by the fusion of asuitable portion of the wrapping material.

A different approach has been followed in e.g., U.S. Pat. Nos.3,391,851, and 5,413,412 where a tear tab is sealed over a perforationline on a heat-shrunk container or on a heat-shrinkable bag. Thedrawbacks in these solutions are mainly related to the risk that theaccidental detachment of the adhered tear tab would expose theperforations thus leading to a loss of vacuum within the package.

Still another approach has been described in U.S. Pat. No. 4,958,735 andprovides for the adhesion of a thick strip of non shrinkablethermoplastic material adhered to the un-shrunk portion of an otherwiseshrunk package, said thick strip bearing a weakness line dividing itinto two manually graspable sections to be used as tear tabs and pulledinto the opposite directions to open the package. While this system hascertain advantages, because for instance there is no need to keep thepackaged product while opening the package and there are no risks forthe packaged product if the tear tabs detach from the bag, themanufacture of such a package would be very complicated and difficult toapply on industrial scale. Furthermore the opening of the package willoccur through a tear of the shrunk film in the longitudinal directionthus destroying the whole container.

There is therefore still a need for flexible containers provided withimproved easy opening means.

OBJECT AND SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide aneasy-openable flexible container of heat-shrinkable thermoplasticmaterial that can be sealed in a tight, hermetic manner, to safelysecure the packaged product, can be employed for the packaging of anytype of products, and can be manufactured easily.

In a first aspect the present invention concerns a plastic bag for themanufacture of an easy-openable, hermetically sealed package, saidplastic bag comprising a multi-layer heat-shrinkable film folded and/orheat-sealed to itself to form a flexible container with two closed sideedges, a closed bottom side edge and an open top side, wherein saidmulti-layer heat-shrinkable film comprises a first outer heat-sealinglayer (a), a second outer layer (b) and directly adhered to theheat-sealing layer (a) an internal layer (c) of a blend of at least tworesin components that are only partially compatible so that said layer(c) will fail when a transversal force of from about 4 to about 9.5N/25.4 mm is applied thereto.

As used herein the term “bag” is inclusive of end-seal (or ES) bags,which have an open top, seamless (i.e., folded, unsealed) side edges,and a seal across the bottom of the bag, transverse-seal (or TS) bags,which have an open top, a seamless bottom edge and each of the sideedges with a seal therealong, and L-sealed (or LS) bags, which have anopen top, a sealed bottom, one transverse-seal along a first side edgeand a seamless second side edge.

In a preferred embodiment of this first aspect, the bag comprises anunsealed area extending beyond one of the factory seals forming the bag,where the front and rear panels of the bag itself are separatelygraspable and usable as tabs to be pulled apart to initiate opening ofthe package. In a most preferred embodiment, said unsealed area beyondthe factory seal is present only along a small portion of the seallength to further facilitate the opening of the bag by concentrating theforce on a limited width of the seal.

Preferably the plastic bag is an ES bag.

In a second aspect the present invention concerns a packaged productcomprising i) an easy-openable, hermetically sealed, plastic bag madefrom a multi-layer heat-shrinkable film folded and/or heat-sealed toitself to form a closed container, wherein said multi-layerheat-shrinkable film comprises at least a first outer heat-sealablelayer (a), a second outer layer (b) and directly adhered to theheat-sealing layer (a) an internal layer (c) of a blend of at least tworesin components that are only partially compatible so that said layer(c) will fail when a transversal force of from about 4 to about 9.5N/25.4 mm is applied thereto, and ii) a product packaged in said bag,said plastic bag having an unsealed area extending beyond one of theseals of the package where the front and rear panels of the bag areseparately graspable and usable as opening tabs.

In a preferred embodiment of said second aspect, the bag is an ES bag.In a more preferred embodiment the bag is an ES bag and the unsealedarea is extending beyond the factory end seal and, even more preferably,said unsealed area is present only along a small portion of the bottomseal length.

Preferably in said second aspect the product is packaged under vacuumand the plastic bag is heat-shrunk around the product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a shrunk bag according to one embodimentof the present invention where the tear tabs extend beyond the sealclosing the bag mouth;

FIG. 2 is a top view of an ES bag according to a preferred embodiment ofthe present invention;

FIG. 3 is a top view of a tubing from which the ES bags of FIG. 2 aremanufactured by using a suitably selected cutting profile;

FIG. 4 is a perspective view of a shrunk bag where the starting bag isthe bag of FIG. 2;

FIG. 5 is a top view of a series of TS bags according to the presentinvention with a side tear tab;

FIG. 6 a to 6 c are representational top view of particular shapes ofpossible tear tabs.

DEFINITIONS

As used herein, the term “film” is used in a generic sense to includeany flexible plastic web, regardless of whether it is film or sheet.Typically, films of and used in the present invention have however athickness of 150 μm or less, preferably a thickness of 140 μm or less,and more preferably a thickness of 120 μm or less.

As used herein, the phrases “inner layer” and “internal layer” refer toany film layer having both of its principal surfaces directly adhered toanother layer of the film.

As used herein, the phrase “outer layer” refers to any film layer havingonly one of its principal surfaces directly adhered to another layer ofthe film

As used herein, the phrase “innermost layer”, when referred to themulti-layer film used in the manufacture of the flexible container,means the outer layer of said multi-layer film which in the end packagewill be closest to the packaged product relative to the other layers ofthe film.

As used herein, the phrase “outermost layer”, when referred to themulti-layer film used in the manufacture of the flexible container meansthe outer layer of said multi-layer film which in the end package willbe furthest from the packaged product relative to the other layers ofthe film.

As used herein, the phrases “seal layer”, “sealing layer”, “heat seallayer”, and “sealant layer”, refer to an outer layer involved in thesealing of the film to itself.

As used herein the term “factory seal” refers to any and all sealsnecessary to convert a film tubing or a flat film into a bag having anopen top. Such seals are made at the bag-making factory and hence areherein termed to be “factory seals”.

As used herein, the term “core” and the phrase “core layer”, refer toany inner film layer that preferably has a function other than servingas an adhesive or compatibilizer for adhering two layers to one another.

As used herein, the phrase “tie layer” refers to any inner film layerhaving the primary purpose of adhering two layers to one another.

As used herein, the phrase “machine direction”, herein abbreviated “MD”,refers to a direction “along the length” of the film, i.e., in thedirection of the film as the film is formed during extrusion and/orcoating.

As used herein, the phrase “transverse direction”, herein abbreviated“TD”, refers to a direction across the film, perpendicular to themachine or longitudinal direction. As used herein, the phrases“heat-shrinkable,” “heat-shrink,” and the like, refer to the tendency ofthe film to shrink upon the application of heat, i.e., to contract uponbeing heated, such that the size of the film decreases while the film isin an unrestrained state. As used herein said term refer to films with afree shrink in each of the machine and the transverse directions, asmeasured by ASTM D 2732, of at least 5% at 95° C.

As used herein, the term “polymer” refers to the product of apolymerization reaction, and is inclusive of homo-polymers, andco-polymers, whereas the term “co-polymer” refers to polymers formed bythe polymerization reaction of at least two different monomers, thusincluding, for example, also ter-polymers.

As used herein, the phrase “heterogeneous polymer” refers topolymerization reaction products of relatively wide variation inmolecular weight and relatively wide variation in compositiondistribution, i.e., typical polymers prepared, for example, usingconventional Ziegler-Natta catalysts.

As used herein, the phrase “homogeneous polymer” refers topolymerization reaction products of relatively narrow molecular weightdistribution and relatively narrow composition distribution. Homogeneouspolymers are structurally different from heterogeneous polymers, in thathomogeneous polymers exhibit a relatively even sequencing of co-monomerswithin a chain, a mirroring of sequence distribution in all chains, anda similarity of length of all chains, i.e., a narrower molecular weightdistribution. This term includes those homogeneous polymers preparedusing metallocene, or other single-site type catalysts, as well as thosehomogenous polymers that are obtained using Ziegler Natta catalysts inhomogenous catalysis conditions.

As used herein, the term “polyolefin” refers to any polymerized olefin,which can be linear, branched, cyclic, aliphatic, aromatic, substituted,or unsubstituted. More specifically, included in the term polyolefin arehomo-polymers of olefin, co-polymers of olefin, co-polymers of an olefinand an non-olefinic co-monomer co-polymerizable with the olefin, such asvinyl monomers, modified polymers thereof, and the like. Specificexamples include polyethylene homo-polymer, polypropylene homo-polymer,polybutene homo-polymer, ethylene-α-olefin co-polymer,propylene-α-olefin co-polymer, butene-α-olefin co-polymer,ethylene-unsaturated ester co-polymer, ethylene-unsaturated acidco-polymer, (e.g. ethylene-ethyl acrylate co-polymer, ethylene-butylacrylate co-polymer, ethylene-methyl acrylate co-polymer,ethylene-acrylic acid co-polymer, and ethylene-methacrylic acidco-polymer), ethylene-vinyl acetate copolymer, ionomer resin,polymethylpentene, etc.

As used herein the term “modified polyolefin” is inclusive of modifiedpolymer prepared by co-polymerizing the homo-polymer of the olefin orco-polymer thereof with an unsaturated carboxylic acid, e.g., maleicacid, fumaric acid or the like, or a derivative thereof such as theanhydride, ester or metal salt or the like. It is also inclusive ofmodified polymers obtained by incorporating into the olefin homo-polymeror co-polymer, by blending or preferably by grafting, an unsaturatedcarboxylic acid, e.g., maleic acid, fumaric acid or the like, or aderivative thereof such as the anhydride, ester or metal salt or thelike.

As used herein, the phrase “ethylene-α-olefin copolymer” refers to suchheterogeneous materials as linear low density polyethylene (LLDPE) witha density usually in the range of from about 0.915 g/cm³ to about 0.930g/cm³, linear medium density polyethylene (LMDPE) with a density usuallyin the range of from about 0.930 g/cm³ to about 0.945 g/cm³, and verylow and ultra low density polyethylene (VLDPE and ULDPE) with a densitylower than about 0.915 g/cm³; and homogeneous polymers such asmetallocene-catalyzed EXACT™ and EXCEED™ homogeneous resins obtainablefrom Exxon, single-site AFFINITY™ resins obtainable from Dow, andTAFMER™ homogeneous ethylene-α-olefin copolymer resins obtainable fromMitsui. All these materials generally include co-polymers of ethylenewith one or more co-monomers selected from (C₄-C₁₀)-α-olefin such asbutene-1, hexene-1, octene-1, etc., in which the molecules of thecopolymers comprise long chains with relatively few side chain branchesor cross-linked structures.

As used herein, the term “adhered”, as applied to film layers, broadlyrefers to the adhesion of a first layer to a second layer either with orwithout an adhesive, a tie layer or any other layer therebetween. Incontrast, as used herein, the phrase “directly adhered” is defined asadhesion of the subject layer to the object layer, without a tie layer,adhesive, or other layer therebetween. As used herein, the word“between”, as applied to a layer expressed as being between two otherspecified layers, includes both direct adherence of the subject layer tothe two other layers it is between, as well as a lack of directadherence to either or both of the two other layers the subject layer isbetween, i.e., one or more additional layers can be imposed between thesubject layer and one or more of the layers the subject layer isbetween.

As used herein the term “gas-barrier” when referred to a layer or to anoverall structure, is used to identify layers or structurescharacterized by an Oxygen Transmission Rate (evaluated at 23° C. and 0%R.H. according to ASTM D-3985) of less than 500 cm³/m²·day·bar.

As used herein the wording “only partially compatible”, when applied tothe polymer components of the blend of layer (c), is intended to referto the individual components of a polymer blend that exhibit asufficient interfacial adhesion to held them in contact with oneanother, at their interfaces, under the intended conditions of use, butthat do not allow the layer containing their blend to maintain itsmechanical integrity when submitted to a given stress, in this case atransversal force in the range of from about 4 to about 9.5 N/25.4 mm.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The multi-layer heat-shrinkable film that can suitably be employed forthe manufacture of the easy-peelable and hermetically sealable bag ofthe present invention contains at least three layers, a first outerheat-sealing layer (a), a second outer layer (b) and, directly adheredto the heat-sealing layer (a), an internal cohesive failure layer (c).

The internal cohesive failure layer (c) comprises a blend of at leasttwo resin components that are only partially compatible and are selectedin such a way that said layer (c) will fail, by an internal rupturesubstantially along a plane parallel to the layer itself, when atransversal force of from about 4 to about 9.5 N/25.4 mm is appliedthereto. Blends of polymer components that can be used for layer (c) arefor instance those described in EP-B-192,131 comprising an ionomer witha melt flow index lower than 5 and a modified ethylene-vinyl acetatecopolymer with a remarkably higher melt flow index, whereby the meltflow indices of the two polymers in said layer (c) differ by at least10; or those described in WO 99/54398 comprising three components, i.e.,a copolymer of ethylene and acrylic or methacrylic acid and inparticular an ionomer, a modified EVA and a polybutene; or thosedescribed in US 2002/0172834 which comprise polybutene, an ionomer, andEVA or an alkyl ester of (meth)acrylic acid in suitable proportions, andthe whole content of these documents is incorporated herein byreference.

Preferred blends for layer (c) are those comprising from about 35 wt. %to about 80 wt. % of a copolymer of ethylene and acrylic or methacrylicacid and in particular an ionomer, from about 15 wt. % to about 30 wt. %of a modified ethylene-vinyl acetate, and from about 2 wt. % to about 50wt. % of a polybutene.

Particularly preferred blends for layer (c) are those comprising fromabout 40 wt. % to about 70 wt. % of an ionomer, from about 15 wt. % toabout 30 wt. % of a modified ethylene-vinyl acetate, such as forinstance an ethylene-vinyl acetate carbon monoxide copolymer, and fromabout 10 wt. % to about 30 wt. % of a polybutene.

Other blends of only partially compatible resins may be employed forlayer (c) provided however they will lead to a breakage of the layerwhen a transversal force of from about 4 to about 9.5 N/25.4 mm isapplied thereto. If layer (c) does fail when a transversal force lowerthan about 4 N/25.4 mm is applied thereto, the bag obtained from thefilm containing such layer might not withstand the pressure exerted inthe loading step by the most conventional automatic loading systems andtherefore there might be leakages in the packages made thereby. If layer(c) fails only when a transversal force higher than about 9.5 N/25.4 mmis applied thereto, the package obtained from the multi-layer filmcontaining such layer (c) will not be easy-openable. The force requiredto break such a layer (c) is measured in accordance with ASTM F88-94using specimens 25.4 mm in width and 300 mm in length made byheat-sealing two strips of a three-layer film where layer (c) issandwiched between two thin polyolefin layers. The two strips aremanually separated until their edges may be fixed respectively into thelower and upper clamps of an Instron dynamometer. The Instron is thenstarted, at a crosshead speed of 30 cm/min with a full-scale load of 2kg, and peel the specimen apart by delaminating layer (c) into twoportions.

The heat-sealable layer (a) of the multi-layer film suitable for themanufacture of the easy-openable bag according to the present inventionwill comprise one or more resins independently selected from the groupconsisting of polyethylene homo-polymer, heterogeneous or homogeneousethylene-α-olefin copolymer, ethylene-vinyl acetate co-polymer,ethylene-ethyl acrylate co-polymer, ethylene-butyl acrylate co-polymer,ethylene-methyl acrylate co-polymer, ethylene-ethyl methacrylateco-polymer, ethylene-butyl methacrylate co-polymer, ethylene-methylmethacrylate co-polymer, ethylene-acrylic acid co-polymer,ethylene-methacrylic acid co-polymer, ionomer and blends thereof in anyproportion. Preferred resins will be ethylene-vinyl acetate copolymers,linear ethylene-α-olefin copolymers, homogeneous or heterogeneous, andblends of two or more of these resins. Particularly preferred resins forthe heat-sealable layer (a) will comprise homogeneous and heterogeneousethylene-α-olefin copolymers with a density comprised between about0.890 and about 0.925 g/cm³, and more preferably with a densitycomprised between about 0.895 and about 0.915 g/cm³ and blends thereofin any proportions. The preferred resins for the heat-sealable layer (a)will have a seal initiation temperature ≦110° C., more preferably a sealinitiation temperature ≦105° C., and yet more preferably a sealinginitiation temperature ≦100° C.

Heat-sealable layer (a) will be the innermost layer in the end packageand is the layer involved in the heat-sealing of the film to itself forthe manufacture of the flexible container and of the end package. Thethickness of said first outer layer (a) would not be higher than 20 μm,preferably not higher than 18 μm and even more preferably not higherthan 15 μm. Typically it will have a thickness higher than 6 μm andpreferably higher than 8 μm to provide for a hermetic seal.Representative thickness values for the heat-sealable layer (a) arepreferably in the range 10-15 μm.

For the other outer layer (b), which will be the outermost layer in theflexible container and in the end package, any thermoplastic materialcan be employed, such as any polyolefin, modified polyolefin or anyblend thereof. Polyamides or copolyamides and polyesters or copolyestersmay also be employed.

The polyamide/copolyamide resins that could be used for the outer layer(b) may be aliphatic nylons e.g., nylon 6, nylon 11, nylon 12, nylon 66,nylon 69, nylon 610, nylon 612, and copolymer nylons including nylon6/9, nylon 6/10, nylon 6/12, nylon 6/66, nylon 6/69, and aromaticnylons, such as 6I, 6I/6T, MXD6, MXD6/MXDI as well as blends thereof.

Preferred thermoplastic polyesters may include those obtained from anacid component comprising an aromatic dibasic acid, such as terephthalicacid or isophthalic acid, and a glycol component comprising an aliphaticglycol, an alicyclic glycol or an aromatic glycol, such as ethyleneglycol, diethylene glycol or cyclohexane dimethanol. The use of aco-polyester, formed starting from two or three species of acidcomponent or/and of glycol component, would be preferred.

Preferred polyolefin resins for the outer layer (b) are ethylenehomo-polymers and ethylene co-polymers. More preferred areethylene-α-olefin copolymers, particularly those with a density of fromabout 0.895 to about 0.935 g/cm³, and more preferably of from about0.900 and about 0.930 g/cm³, ethylene-vinyl acetate copolymers,particularly those with a vinyl acetate content of from about 4 to about14% by weight, ionomers, and their blends.

The thickness of said outer layer (b) typically will depend on thenumber of layers in the overall structure and on their thickness in viewof the total thickness desired for the flexible container. It will thusbe generally comprised between about 2 and about 20 μm, and preferablybetween about 3 and about 15 μm.

According to a preferred embodiment of the present invention themulti-layer film also comprises a core gas-barrier layer (d) thatcomprises at least one gas barrier resin generally selected fromvinylidene chloride copolymers (PVDC), ethylene-vinyl alcohol copolymers(EVOH), polyamides and acrylonitrile-based copolymers. Preferred resinsare typically PVDC, EVOH, polyamides/copolyamides and blends of EVOHwith polyamides/copolyamides.

The most preferred resin is PVDC. This term includes copolymers ofvinylidene chloride and at least one mono-ethylenically unsaturatedmonomer copolymerizable with vinylidene chloride. The mono-ethylenicallyunsaturated monomer may be used in a proportion of 2-40 wt. %,preferably 4-35 wt. %, of the resultant PVDC. Examples of themono-ethylenically unsaturated monomer may include vinyl chloride, vinylacetate, vinyl propionate, alkyl acrylates, alkyl methacrylates, acrylicacid, methacrylic acid, and acrylonitrile. The vinylidene chloridecopolymer can also be a ter-polymer. It is particularly preferred to usea copolymer with vinyl chloride or (C₁-C₈)-alkyl (meth)acrylate, such asmethyl acrylate, ethyl acrylate or methyl methacrylate, as thecomonomers. It is also possible to use a blend of different PVDC such asfor instance a blend of the copolymer of vinylidene chloride with vinylchloride with the copolymer of vinylidene chloride with methyl acrylate.The PVDC may contain suitable additives as known in the art, i.e.stabilisers, antioxidizers, plasticizers, hydrochloric acid scavengers,etc. that may be added for processing reasons or/and to control thegas-barrier properties of the resin.

Ethylene-vinyl alcohol copolymers will be preferably employed when aparticularly good flexibility is required or when a fully coextruded,irradiated structure is manufactured because EVOH withstands irradiationwithout being degraded, up to a very high energy level. It will be usedalone or admixed with a polyamide or copolyamide. Polyamides andcopolyamides can also be employed alone as gas-barrier resins.Particularly preferred in this case are the aromaticpolyamides/copolyamides such as the polyamide formed by polycondensationbetween methaxylyenediamine and adipic acid, the polyamide formed fromhexamethylenediamine and terephthalic acid and/or isophthalic acid andthe copolyamide formed from methaxylyenediamine, adipic acid andisophthalic acid. In general amorphous or semi-crystallinepolyamides/copolyamides are preferred.

Once the gas-barrier resin has been selected, its thickness will be setto provide for the desired oxygen transmission rate (OTR). High barrierstructures will have an OTR below 100 cm³/day·m²·atm and preferablybelow 80 cm³/day·m²·atm and will be particularly suitable for meatpackaging, including fresh red meat and processed meat. Higher OTR willbe preferred for packaging e.g. most of the cheeses where generally OTRof from about 100 to about 400 cm³/day·m²·atm are preferred and fromabout 150 to about 350 cm³/day·m²·atm mostly preferred.

Typically the thickness of the barrier layer ranges from about 2 toabout 10 μm, preferably from about 3 to about 8 μm, and more preferablyfrom about 3.5 to about 7 μm.

Additional layers, such as for instance tie layers, to improveinterlayer adhesion, may be present.

Tie layers may be disposed between the respective layers in case where asufficient adhesion is not ensured between adjacent layers. The adhesiveresin may preferably comprise one or more polyolefins, one or moremodified polyolefins or a blend of the above. Specific, not limitative,examples thereof may include: ethylene-vinyl acetate copolymers,ethylene-(meth)acrylate copolymers, ethylene-α-olefin copolymers, any ofthe above modified with carboxylic or preferably anhydridefunctionalities, elastomers, and a blend of these resins.

If the structure contains tie layers their thickness is generallycomprised between about 0.5 and about 7 μm, and preferably between about2 and about 5 μm.

Other layers may be present in the overall structure such as bulkystructural layers to increase the thickness of the overall structure asdesired, oxygen scavenging layers, additional gas-barrier layers, etc.as known in the art.

Typically the overall thickness of the film for use in the manufactureof the flexible containers of the present invention will be betweenabout 35 and about 120 μm, preferably between about 40 and about 110 μm,and even more preferably between about 45 and about 100 μm.

In all the film layers, the polymer components may contain appropriateamounts of additives normally included in such compositions. Some ofthese additives are preferably included in the outer layers or in one ofthe outer layers, while some others are preferably included in the outerlayers or in one of the outer layers, while some others are preferablyadded to inner layers. These additives include slip and anti-blockagents such as talc, waxes, silica, and the like, antioxidants,stabilizers, plasticizers, fillers, pigments and dyes, cross-linkinginhibitors, cross-linking enhancers, UV absorbers, antistatic agents,anti-fog agents or compositions, and the like additives known to thoseskilled in the art of packaging films.

The films according to the present invention are heat-shrinkable.

Preferably they show a % free shrink in each direction of at least 10%at 95° C. and more preferably a % free shrink at 95° C. higher than 20in at least one direction, and even more preferably a % free shrink at95° C. higher than 20 in each direction.

Preferred films are also those showing a % free shrink higher than 10 ineach direction at a temperature of 90° C. and more preferred thoseshowing a % free shrink higher than 10 in each direction at atemperature of 85° C.

The films according to the present invention can be manufactured by theso-called trapped-bubble process, which is a known process typicallyused for the manufacture of heat-shrinkable films for food contactpackaging. According to said process, the multilayer film is co-extrudedthrough a round die to obtain a tube of molten polymeric material whichis quenched immediately after extrusion without being expanded,optionally cross-linked, then heated to a temperature which is above theTg of all the resins employed and below the melting temperature of atleast one of the resins employed, typically by passing it through a hotwater bath, or alternatively by passing it through an IR oven or a hotair tunnel, and expanded, still at this temperature by internal airpressure to get the transversal orientation and by a differential speedof the pinch rolls which hold the thus obtained “trapped bubble”, toprovide the longitudinal orientation. Typical orientation ratios will becomprised between about 2 and about 6 in each direction and preferablybetween about 3 and about 5 in each direction. After being stretched,the film is quickly cooled while substantially retaining its stretcheddimensions to somehow freeze the molecules of the film in their orientedstate and rolled for further processing.

Cross-linking is typically obtained by passing the flattened tubingthrough an irradiation vault where it is irradiated by high-energyelectrons. Depending on the characteristics desired, this irradiationdosage can vary from about 20 to about 200 kGy, preferably from about 30to about 150 kGy.

Depending on the number of layers in the structure it may be advisableor necessary to split the co-extrusion step: a tube will first be formedof a limited number of layers, with layer (a) on the inside of the tube;this tube will be quenched quickly and before submitting it to theorientation step it will be extrusion-coated with the remaining layers,again quenched quickly, optionally cross-linked, and then passed to theorientation. During the extrusion-coating the tube will be slightlyinflated just to keep it in the form of a tube and avoid that itcollapses. The coating step can then be simultaneous, by coextruding allthe remaining layers altogether, so as to simultaneously adhere all ofthem, one over the other, to the quenched tube obtained in the firstcoextrusion step, or this coating step can be repeated as many times asthe layers which are to be added.

The extrusion-coating step is clearly also required when a film onlypartially cross-linked is desired. As an example, in the case of barrierstructures comprising a PVDC layer which might be degraded/discolouredby irradiation, it might be desiderable to avoid cross-linking of thePVDC layer. In this case the irradiation step will be performed afterthe extrusion of the first group of layers, which would not comprise thePVDC barrier layer, and before the extrusion-coating.

Alternatively, the film according to the present invention may beobtained by flat extrusion (co-extrusion or extrusion coating) andbiaxial stretching by a simultaneous or a sequential tenter process.

Still alternatively the film according to the present invention may beobtained by heat- or glue-laminating separately obtained webs eachcontaining only part of the film sequence of layers.

In a preferred embodiment the film is obtained as a seamless tubularfilm and it is then converted into end sealed (ES) bags by transverselysealing and severing across the seamless tubular film as it lays flat.Alternatively it is converted into transverse sealed (TS) bags byslitting the seamless tubular film along one of its edges and thentransversely seal and severe the thus obtained center-folded film intobags, where the side seals are the sealing and severing seams and thebottom of the bag is the unslit edge of the film.

Other bag and pouch making methods known in the art may be readilyadapted to make receptacles from the multilayer film according to thepresent invention.

The seal(s) along the bottom and/or side edges of the flexiblecontainers of the invention can be at the very edge itself (e.g., sealsof a type commonly referred to in this art as “trim seals” where sealingof e.g. the bottom of one bag will generate the open mouth of thefollowing bag and in such a case the opening tabs to be grasped whenopening of the package is desired, will be created in the bag portionwhich extends beyond the seal closing the open mouth of the package.This embodiment is illustrated in FIG. 1, where 1 is the productpackaged in the bag 2, 3 indicates the trim end seal, 4 is one of theseamless (folded) side edges of the ES bag, 5 is the seal closing themouth of the bag, 6 is the skirt extending beyond seal 5 and 7 a and 7 bare the shaped portions of the unsealed front and rear panels of the bagwhich extend beyond the seal 5. By grasping said two tabs 7 a and 7 band pulling them in laterally opposite directions, it will then bepossible to easily initiate opening of the bag.

However, in general, the factory seals are made using an impulse-typeheat-sealing apparatus which utilizes a heat-sealing bar which isquickly heated and then quickly cooled. The heat-sealing bar may bestraight or possibly shaped, e.g., typically with a curved shape, and isassociated with cutting means, generally parallel to the sealing bar andat a short distance thereof. The heat-sealing means and the cuttingmeans will operate simultaneously and while the heat-sealing means willseal the bottom of one bag, the associated cutting means will create theopen mouth in the following one. This will generate bags where the sealsare spaced inwardly (roughly 0.5-1.5 cm) from the container side and/orbottom edges with a so-called “skirt” of the unsealed front and rearpanels extending beyond the seal and having a dimension corresponding tothe distance between the sealing means and the cutting means. When aflexible container according to a preferred embodiment of the presentinvention is desired, where suitably shaped opening tabs of unsealedmaterial extend beyond one of the factory seals, it will thus besufficient to modify the profile of the cutting means accordingly, whilemaintaining the shape of the heat-sealing bar unchanged.

In both cases, in order to get the best results in terms of easyopenability of the package, the width of the seal, be it a factory sealor the seal made in the packaging process to close the mouth of thepackage, will be less than about 4 mm, more preferably less than about3.5 mm, and typically comprised between about 1 and about 3 mm, e.g., 1to 2 mm.

With reference to the Figures, FIG. 2 is a top view of an ES bagaccording to a preferred embodiment of the present invention. In saidFIG. 10 is the bottom factory seal of the bag, 11 and 12 are the sidefolded edges, 13 is the open mouth of the bag, 14 is the skirt beyondthe bottom factory seal 10 and 15 is that part of the skirt 14 where theunsealed rear and front panels (15 a and 15 b) are suitably shaped to beseparately graspable and thus usable as opening tabs.

FIG. 3 is a perspective view of a lay-flat tubing from which the bag ofFIG. 2 is made. In said FIG. 3, 20 is the seamless tubing in a lay-flatconfiguration, 21 and 22 are the side folded edges of the tubing, 23 isthe end seal of the first bag formed 24, 25 is the skirt of the same bag24 which extends beyond the factory seal, 26 are the opening tabs of bag24, i.e. the area in said skirt 25 where the two unsealed rear and frontpanels will be separately grasped by the user and pulled apart to openthe bag 24, 27 is the mouth of the next bag 28, having the complementaryshape, 29 is the end seal of bag 28, is the skirt of said bag 28, and 31are the opening tabs of bag 28.

FIG. 4 represents a perspective view of a package obtained from a bagaccording to FIG. 2, where the package is shrunk following vacuumizationand heat-sealing of the bag mouth. In said FIG. 4, 40 is the productpackaged, 41 is the bag, 42 is the bottom factory seal, 43 is the skirtextending beyond the bottom factory seal, 44 a and 44 b are the twoopening tabs in the rear and front panels of the shrunk bag that will bepulled in laterally opposite directions to initiate and propagateopening of the bag, 45 is one of the two folded side edges and 46 is thetop seal to close the bag.

FIG. 5 is a top view of a series of TS serrated bags, where 50 is afirst TS bag, 51 is the bottom folded edge, 52 is the open mouth of bag50, 53 and 54 are the side seals of bag 50, 55 are the tabs cut into theside skirt 56 extending beyond the factory side seal 53, 57 is theserration between said bag 50 and the next one 58, 59 is one side sealof bag 58, the other being indicated with numeral 60, 61 is the foldedbottom edge of bag 58, and 62 is the open mouth, 63 is the tab cut intothe side skirt 64 extending beyond the factory side seal 60, and 65 isthe serration between said bag 58 and the next one, not illustrated insaid Figure.

FIGS. 6 a to 6 c are top views of details of ES bags according to apreferred embodiment of the invention wherein alternative shapes of theprofile of the opening tabs are illustrated. In these FIGS. 70 a, 70 b,and 70 c indicate the bottom factory seal of the ES bag, 71 a, 71 b, and71 c are the skirt extending beyond the respective factory seals and 72a, 72 b, and 72 c are alternative shapes for the opening tabs in thesuperposed panels.

As illustrated in these figures, preferably the opening tabs have alimited dimension as this will allow to concentrate the force of theuser to initiate breaking of the seal and therefore opening of the bag,in a direction perpendicular to the direction in which the tabs arepulled. Preferably the opening tabs will have a size suitable to begrasped by the hands of the user and will extend beyond the factory sealonly along a small portion of the factory seal length, typically notexceeding 50%, preferably not exceeding 30% and more preferably notexceeding 20% of the length of the factory seal. With reference to theFIGS. 6 a, 6 b, and 6 c, 73 a, 73 b, and 73 c, indicate the length ofthe factory seal along which the opening tabs respectively extend. Inall those cases the opening tabs are of a size suitable to allowgrasping by the hands of the user, while the length of the factory sealalong which they extend, as well their positioning, may vary to a greatextent.

When the package has to be opened, the end user will keep the package bythe opening tabs and pull them in laterally opposite directions. Theseal adjacent to the tab will then be opened by breaking theheat-sealing layer (a), through the cohesive failure layer (c) in thesealing area, and again breaking the heat-sealing layer (a) below thesealing area but leaving the other layers unaffected. Thus there will beno breakage of the bag but only its opening and no encapsulation. Inpackaging, the product will be loaded into a heat-shrinkable bag made ofthe film of the invention, the bag will normally be evacuated, and theopen end thereof will be closed by heat-sealing, optionally creatingopening tabs in the skirt extending beyond the seal closing the openmouth, if opening tabs extending beyond any of the factory seals are notalready present in the pre-formed bag. Following vacuumization andheat-sealing the packaging material will be heat shrunk by applyingheat. This can be done, for instance, by immersing the filled bag into ahot water bath or conveying it through a hot water shower or a hot airtunnel, or by infrared radiation. The heat treatment will produce atight wrapping that will closely conform to the contour of the producttherein.

An easy-openable, hermetically sealable, bag according to the presentinvention has wide applications, particularly for food packagingapplications, e.g., for the packaging of meat, such as beef and poultry,processed meat, such as ham, mortadella, wurstel, and dairy products,particularly hard cheeses. The bag will have heat-sealing propertiesthat will allow it to survive the process of being filled, evacuated,sealed, closed, heat shrunk, boxed, shipped, unloaded, and stored at theretail supermarket, without loosing the hermeticity, while it will alsohave an easy opening feature that will allow opening of the package byhand, i.e., without using scissors, knives, or other cutting anddangerous devices, when this is desired.

The invention can be further understood by reference to the followingexamples that are merely illustrative and are not to be interpreted as alimitation to the scope of the present invention that is defined by theappended claims.

EXAMPLE

A six layer structure has been prepared by coextrusion of a substrateconsisting of a first (sealing) outer layer (a), an adjacent cohesivefailure layer (c), and a bulk layer (f), followed by quenching of theextruded three layer tubular film, irradiation at 64 kGy and extrusioncoating thereof with a barrier layer (e), an adhesive layer (d) and theouter layer (c). The end extruded tube is then rapidly cooled andbiaxially oriented by passing it through a hot water bath (about 95-98°C.), then inflating to get transverse orientation and stretching to getlongitudinal orientation. The orientation ratios were about 3.5×3.5.

The resins used for the different layers and the partial thicknesses ofeach layer are reported in the Table below.

Thickness layer Resin or resin composition (μm) (a) 80% of metalloceneethylene-α-olefin copolymer 13 with d = 0.902 g/cm³ and MI = 6 g/10 min(Affinity ™PL1280G) and 20% of heterogeneous ethylene-α-olefin copolymerwith d = 0.911 g/cm³ and MI = 6 g/10 min (Stamylex ™ 08-076F by DSM) (c)58% of ionomer (Na methacrylate) with d = 0.94 7 g/cm³ and MI = 1.3 g/10min (Surlyn ® 1601 by DuPont) 22% of Ethylene-vinyl acetate/carbonmonoxide copolymer (24% Vinyl Acetate) with d = 1 g/cm³ and MI = 35 g/10min (Elvalloy ™ 741A) and 20% of polybutylene with d = 0.908 g/cm³ andMI = 1 g/10 min (Polybutene-1 PB8640M by Basell) (f) 50% ofheterogeneous ethylene-α-olefin copolymer 24 with d = 0.920 g/cm³ and MI= 1 g/10 min (Dowlex ™ 2045E by Dow) and 50% of ethylene-vinyl acetatecopolymer (13 wt. % VA) MI = 0.4 g/10 min (Evatane ™ 1003 VN 4 byElf-Atochem) (e) PVDC containing 2 wt. % of plasticisers and 1 wt. % 5of stabilizers (d) 80% of maleic anhydride-modified linear low 6 densitypolyethylene d = 0.913 g/cm³ and MI = 1.7 g/10 min and 20% ofethylene-vinyl acetate modified with ethylene acrylic acid copolymer(Bynel ™ 3101 by Du Pont) (b) co-polyamide nylon 6/66 (85/15)(Ultramid ® C35 by 3 BASF) Melt Flow Indexes (MFI's) are measured byASTM D-1238, Condition E, 190° C./2.16 kg and are reported in grams/10minutes. Unless otherwise specifically indicated, all percentages are byweight.

The % free shrink at 90° C. was measured according to ASTM D 2732 byimmersing for 5 seconds specimens of the structure (10 cm×10 cm) into ahot water bath set at 90° C. and then calculating the % free shrink ineach direction by the equation

% free shrink=(L ₀ −L _(f))/L ₀×100

wherein (L₀ is the initial length of side and L_(f) is the length ofside after shrinking).

The film of Example 1 showed a % free shrink (LD/TD) at 90° C. of 30/40.

The tubing of oriented material was flattened and converted into ES bags350 mm×500 mm, by means of an impulse sealer set at the followingconditions: 160 cycle/min at 16 mA of impulse. The cut profile wasmodified to give bags with the opening tabs as indicated in FIG. 2, thedimensions of said tabs were 80 mm in length and 60 mm in maximum width.

The seal strength of these bags has been measured by means of theParallel Plate Burst Test. In this test each bag was confined betweentwo metal plates based about 10 cm apart within a chamber and inflateduntil its seal fails. In particular each bag was clamped in a fixtureprovided with a hose and the open mouth of the bag was clamped aroundthe hose. Air was pumped through the hose whereby the bag was inflated.The two metal plates restrained the two sides of each bag respectively.For each bag the pressure was increased via the hose at the rate of 1inch of water pressure per second until the seal for that bag burstopen.

The seal strength of these bags has also been measured by the VariablePressure Hot Burst Test that is a test to determine the seal quality ofshrinkable materials at different temperatures. The approach with thistest is to immerse total sealed areas into hot water and after apredetermined dwell time, increase the pressure inside the bag at aconstant rate of approximately 25.4 mm of water per second until theseal fails. The millimeters of water pressure, at the level at which theseal fails, are recorded. Minimum specifications are expressed in mm foreach bag width.

The results obtained with the bags tested in the Parallel Plate BurstTest and in the Variable Pressure Hot Burst Test were respectively 170IOWP and 100 mbar, values both perfectly in line with the productspecification.

The easy-openability of these bags was then tested by loading them witha product (2 kg), vacuumizing in a Cryovac® VC 12 vacuum chamber,heat-sealing the open mouth and finally shrinking in hot (95° C.) water.The shrunk packages were allowed to cool, dried, and then easily openedby hands by pulling the opening tabs in the two laterally oppositedirections.

1: A plastic bag for the manufacture of an easy-openable, hermetically sealed package, said plastic bag comprising a multi-layer heat-shrinkable film folded and/or heat-sealed to itself to form a flexible container with two closed side edges, a closed bottom side edge and an open top side, wherein said multi-layer heat-shrinkable film comprises a first outer heat-sealing layer (a), a second outer layer (b) and directly adhered to the heat-sealing layer (a) an internal layer (c) of a blend of at least two resin components that are only partially compatible so that said layer (c) will fail when a transversal force of from about 4 to about 9.5 N/25.4 mm is applied thereto. 2: The plastic bag of claim 1 which comprises an unsealed area extending beyond one of the seals forming the bag, where the front and rear panels of the bag itself are separately graspable and usable as tabs to be pulled apart to initiate opening of the package. 3: The plastic bag of claim 2 wherein said unsealed area beyond one of the seals forming the bag is present only along a small portion of the seal length. 4: The plastic bag of claim 2 wherein the seal beyond which the unsealed area extends, has a width less than about 4 mm. 5: The plastic bag of claim 1 which is an end seal bag. 6: A packaged product comprising i) an easy-openable, hermetically sealed, plastic bag comprising a multi-layer heat-shrinkable film folded and/or heat-sealed to itself to form a closed flexible container, wherein said multi-layer heat-shrinkable film comprises at least a first outer heat-sealable layer (a), a second outer layer (b) and directly adhered to the heat-sealing layer (a) an internal layer (c) of a blend of at least two resin components that are only partially compatible so that said layer (c) will fail when a transversal force of from about 4 to about 9.5 N/25.4 mm is applied thereto, and ii) a product packaged in said bag, said plastic bag having an unsealed area extending beyond one of the seals of the package where the front and rear panels of the bag are separately graspable and usable as opening tabs. 7: The packaged product of claim 6 wherein the seal in the plastic bag beyond which the unsealed area extends has a width less than about 4 mm. 8: The package of claim 6 wherein the plastic bag is an end seal bag. 9: The package of claim 8 wherein the seal beyond which the unsealed area extends is the factory bottom seal. 10: The package of claim 9 wherein said unsealed area is present only along a small portion of the bottom seal length. 11: The package of claim 6 wherein the product is packaged under vacuum and the plastic bag is heat-shrunk around the product. 12: The plastic bag of claim 1 wherein the blend of resins for internal layer (c) comprises from about 35 wt. % to about 80 wt. % of a copolymer of ethylene and acrylic or methacrylic acid and in particular an ionomer, from about 15 wt. % to about 30 wt. % of a modified ethylene-vinyl acetate, and from about 2 wt. % to about 50 wt. % of a polybutene and preferably comprises from about 40 wt. % to about 70 wt. % of an ionomer, from about 15 wt. % to about 30 wt. % of a modified ethylene-vinyl acetate, and from about 10 wt. % to about 30 wt. % of a polybutene. 13: The plastic bag of claim 6, wherein the blend of resins for internal layer (c) comprises from about 35 wt. % to about 80 wt. % of a copolymer of ethylene and acrylic or methacrylic acid and in particular an ionomer, from about 15 wt. % to about 30 wt. % of a modified ethylene-vinyl acetate, and from about 2 wt. % to about 50 wt. % of a polybutene and preferably comprises from about 40 wt. % to about 70 wt. % of an ionomer, from about 15 wt. % to about 30 wt. % of a modified ethylene-vinyl acetate, and from about 10 wt. % to about 30 wt. % of a polybutene. 